Bandsaws are old and well-known for use in cutting materials such as metal and wood. A bandsaw comprises a few basic parts: a drive wheel, an idler wheel, and a continuous loop or belt of toothed saw blade running over the wheels. The wheels are forced apart to tension the blade, and the drive wheel (which is commonly located below the workpiece) pulls the blade down through the workpiece to make the desired cut. The other side of the blade loop passes outside the workpiece, from the lower drive wheel to the upper idler wheel, over the idler wheel, and is similarly pulled down through the workpiece to continue the cut.
In a large bandsaw operation such as a lumber mill, a bandsaw may have a 4-16-inch wide (and 0.080-inch thick) blade passing over 60-108-inch wheels, with a 20-60-inch throat (the area between the idler wheel and the drive wheel where the blade is pulled down through the workpiece). The blade's basic tension may be on the order of 10,000 pounds, but it may increase by as much as 5,000 pounds when cutting dense lumber at a high feed rate. Saw blade guides may provide further support for the blade above and below the workpiece.
Lumber size control and log feed rate are critical parameters in the economics of bandmill operation. The timber industry uses large bandsaws to reduce logs to useable lumber sizes. Operators manipulate the velocity of the logs through the blade, but frequent out-of-plane deflections in the cut (both laterally and torsionally) due to grain, knots, saw wear, dryness, and mechanical drive-related problems act to decrease both throughput and useable product.
Conventional systems rely on a highly trained operator to monitor each cut and make manual adjustments in feedspeed on the fly. But even with the best currently available equipment, an oscillation of the sawblade in the cut can develop due to excessive feed velocity (“overfeed”). “Snaking” and “washboarding” can also result in poorer tolerance control and production losses. To avoid overfeed, throughput must be conservative, but not so conservative as to reduce yield by sub-optimal throughput. This dilemma is an unsolved problem in the industry.
Size control quality is the amount of variance in size from an expected “target.” In the lumber industry, this is measured generally as a “standard deviation” derived statistically from a set of measurements of deviations of lumber size from the target size. Optimal performance will require feedspeed adjustments faster than an operator can respond, and currently available automated systems do not have feedback means to make timely adjustments in feedspeed. It would seem that automation would offer a means for speeding production throughput, but difficulties have been encountered.
Of the various approaches that have been attempted, initial efforts focused on methods for controlling feedspeed by measuring bandmill power consumption. These methods have generally failed at industrial scale because the inertial resistance to any change in RPM of the bandsaw wheels, which may be 6 ft in diameter, results in lags in response time relative to deviation of the blade from a true cut. Thus power control loops have proven sluggish in response time and are unsatisfactory because of the “flywheel effect.”
A number of inventions, including co-owned U.S. Pat. No. 6,681,672 to Myrfield and U.S. Pat. No. 4,926,917 to Kirbach have employed optical, video, or laser means to “map” the saw cut so as to optimize process parameters. Other art is described in which an eddy current transducer mounted on the top sawguide, or a laser beam, and is used to detect deflection during a cut. Use of power consumption measurement has also been attempted. Again, all these are lagging indicators of deviation and are ineffective in preventing wood losses due to poor target control. Transducers are mounted on the top sawguide because the bottom sawguide is continuously showered by sawdust during a cut and as boards are cut away, the transducer is frequently hit and broken or dislodged.
Similar issues are unresolved in U.S. Pat. Nos. 5,694,821 and 6,382,062 to Smith. The systems described above all teach measurement of displacement of the sawblade proximate to the point where a tooth enters the cut. But any displacement of the saw blade at the top of the cut is preceded by a change in one or more forces at the bottom of the cut. Displacement of the blade inherently cannot occur until the tooth has sawn through the cut. Therefore, displacement measurements made above the cut are “trailing” or “lagging” measurements. Lagging signals are inadequate for closed loop control of velocity, particularly at higher feedspeed where incipient deviation occurs in microseconds. Thus all these systems are ineffective in supporting a rigorous log size control quality program for optimal yields per log.
More recently, Myrfield (U.S. patent application Ser. No. 14/556,139, co-owned by Myrfield) has proposed a force sensing system operatively coupled to the bottom sawguide. Force is measured directly and instantaneously; not as blade deviation, but through a mechanical linkage whereby force is transmitted directly to a load cell. In practice this innovation has been found to be in need of improvement because load cell-type electromechanical sensors in highly cyclic use were found to fail quickly. It is believed that the short working life is due to mechanical wear on the sensor in this direct linkage. Load cells with strain gauge sensors of the foil type or the piezoelectric type may also be damaged during setup due to the fine adjustments necessary to preload a cell with a very small displacement. Thus, the limited sensor lifetime and delicacy of the sensor packages currently available has proved to be a hurdle to commercial acceptance.
Given this background, there is a need in the art for a bandsaw feed rate controller and system with force sensing devices that are both robust and provide the needed sensitivity to incipient deviation; sensitivity that is greater or equal to the load cell systems developed earlier by Myrfield (U.S. patent application Ser. No. 14/556,139).